File I/O with arrays

This is a discussion on File I/O with arrays within the C++ Programming forums, part of the General Programming Boards category; Yet another newbie-ish problem.... this time slightly more advanced. I'm having a bit of trouble reading information as text from ...

  1. #1
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    File I/O with arrays

    Yet another newbie-ish problem.... this time slightly more advanced.

    I'm having a bit of trouble reading information as text from a file.... The information involves three different sets of integers, two normal, and one an array, like so:

    Code:
    100
    30
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111131111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111131111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111222222221111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111131111111111111111111111111222222221111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111131111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111113111111111111111111111111
    1111111111111111113111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111311111111111111111111111111111111111111111111111111
    1111111111111111111111111111131111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111311111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
    Yes, it's a lot, but I'm trying to hard-code something for testing.

    My file I/O operation currently reads like so:-

    Code:
    int location::fileread(char filename[10])
    {
        ifstream a_file ( filename );
        if ( !a_file.is_open() ) {
        cout<<"File could not be opened./n";
        return 0;
          }
    else {
        a_file>>mapx;                              //read the first value, the x-value
        a_file>>mapy;                              //read the second value, the y-value
        a_file>>terrain[mapx][mapy];      //store the long section of 1s, 2s, and 3s in terrain[100][30]
        return 1;}
    }
    This results in me getting a blank screen with no information whatsoever. I don't know if this means that the x and y values are being read but the array isn't, or if the whole lot is horribly wrong (the results would, I think, be pretty much the same). I'm pretty sure the problem lies in the array. Does instreaming read a single character? Multiple characters? The entire line? Is there any way to alter this?

    It's probably a really stupid question, but any help would be greatly appreciated!

  2. #2
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    Erm, why are you returning 1 after the else statement as that means that there is an error. Return 1 if the file can't be opened.

  3. #3
    and the hat of wrongness Salem's Avatar
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    > a_file>>terrain[mapx][mapy]
    This refers to the element just off the end of the array

    Maybe
    Code:
    for ( int r = 0 ; r < mapy ; r++ ) {
      for ( int c = 0 ; c < mapx ; c++ ) {
        a_file>>terrain[r][c];
      }
    }
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    To be honest, I just prefer it that way.... the main() setup goes something like so:

    Code:
    int main()
    {
        char mapf[10];
        cout<<"Enter map file:\n";
        cin>>mapf;
        if (locate.fileread(mapf) == 1){
        clrscr();
        charx = mapx/2;
        chary = mapy/2;
        mapprint(mapx, mapy);
        do {
            input();
            }
            while (input() != 0) ;
            clrscr();
            }
            else {
        cursorpos(0,0);
        cout<<"Goodbye!";}
    }
    I just have a thing about 1 being 'true' and 0 being 'false'. It's not that much trouble to change, though, if you think I should.

  5. #5
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    Hmm, thanks for the advice Salem, but it's still not loading correctly . Maybe I should figure out how the C commands work rather than mess around with the C++ commands... although the C version seems so much more complex.

    EDIT:
    I've figured out how to manage it, rather than storing each variable in a two dimensional way in the file itself, I have to store it one on top of the other (like
    1
    1
    1
    1
    1
    1
    1
    etc...) and then read it like that. Unless there's some way to integrate strings and integers.... anyway, for the time being, problem solved.
    Last edited by 20,000leeks; 09-17-2006 at 07:10 AM.

  6. #6
    Code Goddess Prelude's Avatar
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    >Maybe
    Code:
    for ( int r = 0 ; r < mapy ; r++ ) {
      for ( int c = 0 ; c < mapx ; c++ ) {
        a_file>>terrain[r][c];
      }
    }
    Maybe not. It looks like all of the digits are intended to be unique entities but they're not separated by whitespace. a_file>>terrain[r][c] is unlikely to work as expected in this situation. Something more like this would work if I'm judging the file properly:
    Code:
    a_file>> first;
    a_file>> second >> ws;
    
    for ( int i = 0; i < mapy; i++ ) {
      for ( int j = 0; j < mapx; j++ )
        terrain[i][j] = a_file.get() - '0';
    }
    My best code is written with the delete key.

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    Hmm, it *almost* works. I think because there's a white space on the end of each line, that makes things a little more complex, so I tried:

    Code:
    for ( int i = 0; i < mapy; i++ ) {
           for ( int j = 0; j < mapx; j++ )
            if ( j < mapx){
            terrain[j][i] = a_file.get() - '0';
            }
            else a_file>> terrain [j][i];
           }

    but it still comes out on a slant (like this

    Code:
     .................................................
    . ................................................
    .. ...............................................
    ... ..............................................
    .... .............................................
    ..... ............................................
    ...... ...........................................
    ....... ..........................................

  8. #8
    Frequently Quite Prolix dwks's Avatar
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    You need to read in the '\n', I think.
    dwk

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  9. #9
    Code Goddess Prelude's Avatar
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    Ah, so there are linebreaks in the file itself. Okay, how about this:
    Code:
    for ( int i = 0; i < mapy; i++ ) {
      for ( int j = 0; j < mapx; j++ ) {
        char ch;
    
        if ( !a_file.get ( ch ) ) {
          // Handle an input error
        }
    
        if ( ch != '\n' )
          terrain[i][j] = ch - '0';
      }
    }
    My best code is written with the delete key.

  10. #10
    Frequently Quite Prolix dwks's Avatar
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    You could just do this:
    Code:
    for ( int i = 0; i < mapy; i++ ) {
      for ( int j = 0; j < mapx; j++ )
        terrain[i][j] = a_file.get() - '0';
    
      while(a_file.get() != '\n' && !a_file.bad());
    }
    .bad() might not be the best function to use however.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


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  11. #11
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    Hmm, I tried
    Code:
    for ( int i = 0; i < mapy; i++ ) {
           for ( int j = 0; j < mapx; j++ )
           if (a_file.get()!='\n'){
            terrain[j][i] = a_file.get() - '0';
            }
            else
            {j = mapx;
            }
    but that seemed to cut out parts of the file.

    Code:
    for ( int i = 0; i < mapy; i++ ) {
      for ( int j = 0; j < mapx; j++ )
        terrain[i][j] = a_file.get() - '0';
    
      while(a_file.get() != '\n' && !a_file.bad());
    }
    didn't load the map at all for some reason :S

  12. #12
    Code Goddess Prelude's Avatar
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    >You could just do this:
    Handling the linebreak outside of the inner loop is definitely a better solution, but I'd still want some error checking:
    Code:
    for ( int i = 0; i < mapy; i++ ) {
      for ( int j = 0; j < mapx; j++ ) {
        char ch;
    
        if ( !a_file.get ( ch ) ) {
          // Handle an input error
        }
    
        terrain[i][j] = ch - '0';
      }
    
      a_file.ignore();
    }
    My best code is written with the delete key.

  13. #13
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    Aha! Problem solved! Thanks a lot, I'd probably have resorted to using a single-column file otherwise....

  14. #14
    Frequently Quite Prolix dwks's Avatar
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    I forgot about ignore() . . .

    I'm sure you've fixed this by now, but here's an earlier line you posted:
    Code:
    cout<<"File could not be opened./n";
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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  15. #15
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    Ah, yes, I noticed that one about five minutes after posting it.... I guess there's an upside to uncomplicated applications, you catch typos faster.

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